GENEVA, Aug 22 (Reuters) - At least 191,369 people have been killed in Syria's conflict through April, more than double the figure documented a year ago and probably still an under-estimate, the United Nations human rights office said on Friday.

The U.N. report, based on data from four groups and the government that were cross-checked, reflects a reality of killing and torture while the civil war has "dropped off the international radar", U.N. High Commissioner for Human Rights Navi Pillay said.

"With additional killings reported from earlier periods, in addition to the new killings that have taken place, the total is more than double the number documented a year ago," Pillay said in a statement.

"Nevertheless, as the report explains, tragically it is probably an underestimate of the real total number of people killed during the first three years of this murderous conflict."

An additional 51,953 killings that were reported lacked sufficient information and were excluded from the analysis, said the report issued in Geneva. A further significant number may not have been reported by any of the five sources, it added.

The highest number of documented killings were recorded in Rural Damascus province(39,393), followed by Aleppo (31,932), Homs (28,186), Idlib (20,040), Daraa (18,539) and Hama (14,690).

Pillay repeated her call for referring alleged war crimes and crimes against humanity committed by all sides in Syria's conflict to the International Criminal Court (ICC).

"The killers, destroyers and torturers in Syria have been empowered and emboldened by the international paralysis," she said.

"It is essential governments take serious measures to halt the fighting and deter the crimes, and above all stop fuelling this monumental, and wholly avoidable, human catastrophe through the provision of arms and other military supplies," she said. (Editing by Ralph Boulton)